Diet and seed removal rates by the harvester ants Pogonomyrmex rastratus and Pogonomyrmex pronotalis in the central Monte desert, Argentina

Abstract

Granivorous animals, through seed consumption, may have an important infl uence on plant abundance, distribution and species composition in desert ecosystems. The aims of this study are twofold: to quantify the diet of Pogonomyrmex rastratus (Mayr) and Pogonomyrmex pronotalis (Santschi), and to estimate seed removal per colony of both species throughout their activity season (October-April) in the central Monte desert, Argentina. Both species rely heavily upon seeds, which account for 87-94 % of the items carried to the nests. Their diets are similar, consisting mainly of grass seeds, which represent more than 93 % of the seeds. Among them, three species predominate: Aristida spp., Trichloris crinita and Pappophorum spp. Seasonal variations as well as seed species richness in the diet are also similar between species. However, their food-handling behaviour differs: most caryopses carried by P. pronotalis bear bracts whereas most caryopses carried by P. rastratus lack them. Seed removal per colony by P. rastratus (6 × 10 4 seeds/colony) and by P. pronotalis (5 × 10 4 seeds/colony) throughout the season is similar to the one reported for P. occidentalis in North America. However, seed removal per hectare, which could be estimated for P. rastratus (8.3 × 10 5 seeds/ha), is lower than removal rates reported for the North American species P. barbatus, P. desertorum, P. rugosus and P. californicus, probably because P. rastratus has lower activity levels and smaller colonies than the North American studied species.

Arthropod-Plant Interactions

“…Inside the nests we found grass caryopses with and without bracts. Harvester ants may keep the seeds intact in the nest until consumption, when bracts are removed and the seeds eaten (see also Pirk and Lopez-de-Casenave 2006). No seed species collected by P. naegelli possessed an elaiosome (i.e., nutritious seed appendage typical of specialized antdispersed species known as myrmecochores; see Beattie 1985).…”

“…All seeds found inside nest chambers were collected and conserved dry in labeled tubes for further identification and were weighted in the laboratory. Dry fruits producing only one seed, such as caryopses or achenes, were considered seeds (see Whitford 1978;Pirk and Lopez-de-Casenave 2006).…”

“…3,4). Seasonal variation in the diet entailing the consumption of non-seed items has been observed in a number of harvester species, including Pogonomyrmex (e.g., Whitford 1978;MacKay 1981;Mehlhop and Scott 1983;Pirk and Lopez-de-Casenave 2006;Pol et al 2011). Few studies, however, have shown quantitatively such a marked seasonal shift in granivory and carnivory as recorded for P. naegelii (Tevis 1958;Whitford et al 1976;Kugler and Hincapié 1983;Pirk et al 2009a).…”

“…5) corroborate numerous other studies reporting that grasses head the list of most consumed species by harvester ants (Whitford et al 1976;Taber 1998;MacMahon et al 2000;Pirk et al 2009a;Pol et al 2011;and included references). Indeed, a number of attributes may account for the prevalence of grass seeds in the diet of harvester ants, including their predictability in space and time, absence of secondary compounds, as well as adequate morphology and small size facilitating transport (see Carroll and Janzen 1973;Pulliam and Brand 1975;Crist and MacMahon 1992;MacMahon et al 2000;Pirk and Lopez-de-Casenave 2006).…”

A New Protocol Using Artificial Seeds to Evaluate Dietary Preferences of Harvester Ants in Semi-arid Environments

“…However, most of Pogonomyrmex species exhibit particular Red de Ecoetología, Instituto de Ecología A.C., Xalapa, Veracruz, Mexico RESEARCH ARTICLE -ANTS preferences in their seed intake, predating some plant species more than others (i.e., differential seed predation) (Taber, 1998;MacMahon et al, 2000;Rico-Gray & Oliveira, 2007). On the other hand, in the wide geographical distribution of Pogonomyrmex species, there is not a consensus about their seed preferences and which are the main factors influencing the seed selection by harvester ants (MacMahon et al, 2000;Pirk & De Casenave, 2006;Belchior et al, 2012;Schmasow & Robertson, 2016). In addition, in arid environments, where harvester ants are found, the landscape is highly heterogeneous (in terms of vegetation, climatic conditions, topography and geology) in both spatial and temporal scales (Bestelmeyert & Wiens 2001;Rietkerk et al, 2002;Whitford, 2002), which has led to the implementation of particular methodologies conceived to be used in a given arid environment (since they cannot operate in the same way and need to be readjusted for other localities (e.g.…”